This invention relates in general to the bonding of metal parts together and, more specifically, to the diffusion brazing of titanium based alloys together, such as face sheets and honeycomb cores.
Brazing is a very old and well known process for joining metal parts to each other. Typically, a layer of a metal having a lower melting temperature than the parts is interposed between the faying surfaces of the parts. The assembly is then heated to at least the melting temperature of the interlayer (or to the melting point of the interlayer-base metal eutectic) which melts, wets the faying surfaces and generally diffuses somewhat into those surfaces. While small parts can often be brazed using a torch and wire forming the interlayer as it is melted, larger and more complex structures are generally brazed in furnaces where temperature, pressure and atmosphere may be easily controlled. Upon cooling, a strong continuous joint is formed. Bonding aluminum or copper parts by such techniques is described, for example, by Bose et al. in U.S. Pat. No. 4,497,430 and Wells et al. in U.S. Pat. No. 3,581,382.
Attempts have been made to diffusion bond parts together without an added metal interlayer by very carefully cleaning the faying surfaces and pressing them forcefully together in an inert atmosphere or vacuum while heating the parts to a high temperature which is lower than the melting temperature of the parts. This process is discussed by Woodward in U.S. Pat. No. 3,957,194. This technique has not proved practical with large delicate assemblies, such as honeycomb core structural panels because of the difficulty in obtaining the necessary high surface cleanliness and maintaining uniform pressure during bonding and the probability of core crushing due to the high pressures involved.
Titanium aluminide alloys are very desirable for use in honeycomb core sandwich panels because of their high temperature strength and stiffness-to-weight ratios. It is impractical to bond these panels using diffusion bonding as the typical bonding pressures required (200 to 5000 psi) would crush the delicate honeycomb core or in the case of thick sections require expensive and complex tooling to provide the high pressure.
Titanium aluminide has been bonded using 0.001 to 0.002 inch titanium-copper-nickel brazing foil. This has several disadvantages in that the multi-layer laminate is difficult to roll down to the required thickness. The use of such braze foil in thicknesses greater than about 0.001 inch can result in an excess of brazing liquid that may erode the thin gauges of core foil (0.001 to 0.005 inch) typically used in honeycomb core.
Other possible bonding agents do not have the required high temperature resistance, toughness and bond line strength required for many applications. Other techniques for bonding titanium use complex and expensive interlayer foils, such as that described by Parker in U.S. Pat. No. 3,981,429.
Thus, there is a continuing need for an improved method of bonding titanium aluminide based alloys in applications such as the manufacture of honeycomb core panels.